1. Field of the Invention
The present invention relates to an apparatus for displaying a frequency received by a radio receiver. More specifically, the present invention relates to an apparatus for displaying in a digital manner a frequency being received by a radio receiver by counting the local oscillation frequency pulses obtainable from a local oscillator of the radio receiver.
2. Description of the Prior Art
It has been proposed that the frequency of a signal received by a radio receiver be displayed in a digital manner. For example, U.S. Pat. No. 3,244,983, issued Apr. 5, 1966 to Robert J. Ertman and entitled "Continuously Tunable Direct Reading High Frequency Converter" discloses that a numerical value associated with the intermediate frequency is preset in a preset counter and the local oscillation frequency is evaluated by counting the local oscillation frequency output by means of the said preset counter, whereupon the count value is displayed in a digital manner. On the other hand, the actual intermediate frequency of the respective radio receivers, as manufactured, is determined by the accuracy of adjustment of the intermediate frequency transformers, the frequency characteristic of ceramic filters used in the intermediate frequency amplifiers, and the like. However, it is usually not possible to achieve exactly the same adjustment of the intermediate frequency transformers or obtain ceramic filters of exactly the same frequency characteristics in all the radio receivers, as manufactured, and thus some deviation of the actual intermediate frequency of the intermediate frequency stage unavoidably results among the radio receivers, as mass-produced. Hence, it is appreciated that only if a predetermined numerical value associated with the prescribed intermediate frequency is fixedly preset in the preset counter, then some frequency deviation occurs in indicating of the frequency being received. Thus, ideally it is necessary to correct the numerical value being preset in the preset counter in association with the deviation of the intermediate frequency of the intermediate frequency stage of the respective radio receivers by virtue of the deviation of adjustment of the intermediate frequency transformers, the frequency characteristics of the ceramic filters and the like. However, since it was conventionally adapted such that the preset numerical value was read out from a masked read only memory, for example, it was impossible to make such correction of the preset value. Thus, it was required that the accuracy of adjustment of the intermediate frequency transformers be strictly controlled or that ceramic filters of an accurate frequency characteristic were selected, which caused an increase of the cost.
On the other hand, Japanese Patent Publication Gazette No. 44167/1977, as published Nov. 5, 1977 for opposition, discloses a dial display in a radio receiver, wherein in order to avoid .+-.1 count error occurring in the least significant digit of the displayed number counters are used the number of which is larger by at least one digit than the number of digits being displayed so that the additional counter is allotted for the digit position less significant than the least significant digit of the number being displayed and, for example, the number "5" is preset in the additional counter, whereby the number being displayed is rounded at the digit less significant than the least significant digit so as to count fractions of 5 and over as a whole number and to disregard the rest, with the result that an error is eliminated from an apparent digital display. Even such prior art display involves a problem to be solved and hence leaves room for improvement. More specifically, according to the above described approach, the preset value per se is not corrected in association with the deviation of the actual intermediate frequency of the receivers from the prescribed intermediate frequency but instead the displayed number is merely rounded at an additional digit position less significant than the least significant digit of the number being displayed, thereby to achieve an apparently corrected digital display of the frequency being received. Therefore, if and when deviations of the adjustment of the intermediate frequency transformers or the deviation of the frequency characteristic of the ceramic filters used in the intermediate frequency amplifiers exceeds the range in which any error can be eliminated in accordance with the above described approach disclosed in the referenced Japanese Patent Publication Gazette, then it follows that such deviation appears as an error in indicating the frequency being received which is made in the conventional display. In other words, the approach disclosed in the above referenced Japanese Patent Publication Gazette merely broadens a permissible error range in which an error of adjustment of the intermediate frequency transformers or an error of the frequency characteristic of the ceramic filters is permissible and totally fails to provide any scheme for correcting the preset value per se fundamentally in association with such deviation.